1. Field of the Invention
Disclosed herein is a stent that is radially deformed in vivo after implantation of the stent in a bodily lumen.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial stent that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically-shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the cross-sectional area of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. Radial strength and rigidity, therefore, may also be described as, hoop or circumferential strength and rigidity.
Additionally, the stent should also be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
The structure of a stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is conventionally designed so that the stent can be radially contracted (to allow crimping) and radially expanded (to allow deployment). A conventional stent is allowed to expand and contract through movement of individual structural elements of a pattern with respect to each other. Such movement typically results in substantial deformation of localized portions of the stent's structure. The pattern of the stent is designed to maintain the longitudinal flexibility and radial rigidity required of the stent. Longitudinal flexibility facilitates delivery of the stent and radial rigidity is needed to hold open a bodily lumen.
A stent may be constructed of polymeric material. Polymeric stents require adequate circumferential strength and radial rigidity. Inadequate circumferential strength potentially contributes to a relatively high incidence of recoil of polymeric stents after implantation into vessels. The requirement of high strength and rigidity is seemingly at odds with the need for flexibility during delivery. The movable structural elements in the stent pattern provide some flexibility.
One potential problem with polymeric stents is that their struts can crack during crimping and expansion. The localized portions of the stent pattern subjected to substantial deformation tend to be the most vulnerable to failure. Therefore, it would be desirable to have a method of treating a bodily lumen with a polymeric stent in which the stent has adequate flexibility during delivery, high radial strength and rigidity after deployment, and that is relatively free of localized regions of high deformation susceptible to failure.